Clemens, in U.S. Pat. No. 3,842,194, discloses an information recording and playback system which utilizes variable capacitance. In one configuration, information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. The disc record is overcoated with a conductive layer followed by a dielectric layer and in turn by a lubricant layer. Recent developments have resulted in the use of a conductive disc material which eliminates the need for discrete conductive and dielectric layers. For example, groove widths of about 2.5 micrometers and groove depths of about 0.5 micrometer may be used. During playback a pickup stylus fabricated from a dielectric support element having a tip about 2 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometer thick, engages the groove as the record is rotated on a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the prerecorded information.
Keizer, in U.S. Pat. No. 4,104,832, discloses a method for forming a keel-tipped stylus. A tapered dielectric support element made from a hard dielectric material, such as diamond, is placed in contact with an abrasive lapping disc having a deep trapezoidally-shaped, coarsely pitched, spiral groove. Relative motion is established between the support element and the lapping disc. The lands of the lapping disc lap the shoulders of the keel-tipped playback stylus and the side walls of the groove serve to form the slightly tapered side surfaces of the constricted terminal portion of the stylus.
In the aforementioned Keizer method U.S. Pat. No. 4,101,832 the abrasive lapping disc may be formed, if desired, by employing optical recording techniques. To that end, Keizer describes a flat, smooth, copper-clad substrate that is coated with a thick coating of photoresist, for example, approximately 4.0 micrometers in thickness. The photoresist-coated substrate is then exposed to a focused laser beam, for example, about 2.0 micrometers in diameter at the focus, along a spiral track which has a pitch that is considerably coarser than that of a disc record. For example, the coarse pitch might be 6-10 micrometers. The exposed photoresist is developed to form a spiral groove in the photoresist. The developed photoresist layer is coated with a conductive metal layer, generally 0.12 micrometer of gold, applied by evaporation from a liquid in a vacuum chamber. Metal stampers, made from the substrate by standard plating techniques, may then be used to press plastic lapping discs having a groove of substantially the same shape and size as the groove which was formed in the photoresist layer. An abrasive material, such as SiO.sub.x, wherein x is between 1 and 2, may be glow discharge deposited on the plastic disc to form an abrasive layer suitable for lapping a keel-tipped playback stylus. Kaganowicz in his U.S. Pat. No. 4,328,646, issued May 11, 1982 and entitled, "Method for Preparing an Abrasive Coating," discloses, abrasive SiO.sub.x coating which may be advantageously employed.
Kaganowicz in U.S. Pat. No. 4,369,604, issued Jan. 25, 1983 entitled, "Method and Apparatus for Mechanically Preparing Stylus Lapping Discs," discloses a method and apparatus for electromechanically preparing lapping discs. A deep groove is mechanically cut into a metal substrate surface with a diamond cutting tool. The diamond cutting tool tip has a shape complementary to the shape of the groove to be formed in the surface of the substrate. During the mechanical cutting operation, the tip of the stylus is introduced into the metal surface to a depth sufficient to cut the groove to the desired depth across the surface of the metal substrate. A signal track is mechanically cut on the lapping disc surface by a cutting stylus to provide a signal for monitoring the subsequent lapping of the tip (i.e., the keel) of the playback stylus. The signal can be cut into the spiral groove or cut into the land between the spiral groove of the lapping disc. The pitch of the groove and either of the two forms of signal tracks are equal. The signal from the track is sensed during the lapping operation to indicate when the desired width of the shoulder of the tip is achieved.